Not just that, sometimes people simply like being scared, when they're not actually scared. And that doesn't even have to be in a sexual context. But, regarding it in a sexual context, it's called a fear kink. This is not a common kink, and requires a lot of healthy communication, but these are fictional characters.
Besides, I don't think Hitomi is necessarily fearful of Kamiya, but rather at being overwhelmed by him, physically. Sexually. Hesitation, anticipation, fear that his mind will break. Though, when it comes to sex, some people want their minds to break, to be overwhelmed, even if they're scared of it. So... Idk man, the human mind is complex, but that's probably why this Mangaka makes it seem like their ukes are 'fearful', in a way, of the Seme. It's... It contributes to the whole- sort of- Predator/Prey vibe that *maaany* people are weak for in fictional stories. And mostly fictional stories. There are a lot of things some people find hot in a fictional scenario or story that they'd never find attractive in real life.
if hitomi likes kamiya so much and wants him to do whatever to him, why tf does he look terrified in every panel. it feels like the mangaka was going for a psychological angle but.. tried to make it out like it's a cute fluffy manga? like what? also their size difference is icky.